If you have two projects with similar and overlapping names, such as "Plasma" and "Plasma on Wayland" then it seems that both Herald and the tags widget on reviews get confused.
As such if you try to add "Plasma on Wayland" to a review which already has "Plasma" on it then you end up removing "Plasma" even though this wasn't what you intended to do. The opposite is also true (adding "Plasma" when it already has "Plasma on Wayland" will remove "Plasma on Wayland")
This can be seen in the history of https://phabricator.kde.org/D10235 where you can see the tug-of-war Herald is engaged in between two different rules, and with individual people. Both rules are only supposed to add Projects to the review, but also appear due to this issue have the effect of removing the other Project when it's triggered.